This relates to data communications apparatus and, more particularly, to apparatus for demodulating Differential Phase Shift Keying (DPSK) modulated signals.
Data transmission over voice frequency communication channels is generally accomplished via data sets which employ modulation and demodulation processes. More specifically, binary digits which are to be transmitted are initially converted to symbol signals (e.g., two bits per symbol). The symbol signals are then appropriately modulated with a carrier signal and the modulated signals are sent over the communication channel. At a receiving data set, the modulated signals are appropriately demodulated to recover the binary digits.
One well-known modulation approach, for example, is Pulse Amplitude Modulation (PAM). In a PAM signal EQU s(t)= a(t)cos .omega..sub.c t, (1)
the term a(t) which defines the amplitude of the carrier contains the symbol information. Demodulation of the PAM signal of Equation (1) can be accomplished simply by multiplying the signal s(t) by a demodulating carrier having a radian frequency .omega..sub.c and by applying the multiplied signal to a low-pass filter.
For coherent demodulation of amplitude modulated signals, D. A. Spaulding in U.S. Pat. No. 3,761,829, issued Sept. 25, 1973, describes the use of a Hilbert transform filter followed by sampling at the symbol rate and further followed by analog-to-digital conversion and digital multiplication by appropriate carrier signals. The Spaulding circuit, however, is only effective for amplitude modulation.
Another well-known modulation approach is Differential Phase Shift Keying modulation (DPSK). In a DPSK signal ##EQU1## THE TERM G(T) IS A Nyquist pulse, T is the symbol period (baud period), .omega..sub.c is the transmitted signal's carrier frequency and .phi..sub.n is the phase angle which represents the symbol information (via the differential angle .DELTA..phi..sub.n = .phi..sub.n - .phi..sub.n.sub.-1).
A situation of particular interest presents itself when only four symbols are to be transmitted and where, therefore, only four phase angles are necessary for DPSK modulation. Advantageously, the phase angles employed are .+-. .pi./4, .+-. 3.pi./4. In Principles of Communication, by R. W. Lucky et al, McGraw-Hill 1968, FIG. 9.7 on page 257 depicts a four-phase DPSK demodulation circuit (called a comparison detector) employing the above phase angles. The described circuit comprises a front-end band limiting filter branching to four signal paths, a 90.degree. phase shifter (a Hilbert filter) followed by a delay of length T in one path, a delay of length T in another path, means for appropriate multiplication, and means for post detection filtering. Mathematically, the operations performed by the comparison detector are LPF {s(t).sup.. s(t+T)} for developing one output, and LPF {s(t) .sup.. H[s(t+T)]} for developing a second output, where H designates Hilbert filtering and LPF designates low-pass filtering. Because of the multiplications involved, the comparison demodulator of FIG. 9.7 doubles the input signals' bandwidths and consequently fails by causing interfering spectral overlap when the transmitted baseband signal has a wide bandwidth compared to the carrier frequency. This interference cannot be effectively eliminated by post-multiplication filtering. In fact, the interference is increased in a digital implementation of the FIG. 9.7 circuit through the phenomenon of aliasing if the processing clock in the digitally implemented circuit is not high enough.